In recent years, active matrix liquid crystal display apparatuses having thin film transistors (TFTs) as the switching elements have achieved widespread use. In this type of liquid crystal display apparatus, a liquid crystal layer is interposed between a TFT substrate and a counter substrate. The TFT substrate includes a plurality of gate lines running in parallel and a plurality of source lines running in parallel in the direction crossing the gate lines at right angles. Pixel electrodes, which constitute respective pixels, are provided to correspond to the respective crossings of the gate lines and the source lines, forming a matrix as a whole. A TFT is provided for each of the pixel electrodes, so that the gate electrode, source electrode and drain electrode of the TFT are respectively connected to the gate line, the source line and the pixel electrode. A storage capacitor is formed for each pixel electrode with one terminal connected to the pixel electrode. The other terminal of the storage capacitor is connected to the adjacent gate line (CS-on-gate type) or a storage capacitor line (CS-on-common type). The counter substrate includes a counter common electrode. A liquid crystal (LC) capacitor is formed between the pixel electrode and the counter common electrode, and the LC capacitor and the storage capacitor constitute a pixel capacitance.
In general, a liquid crystal display apparatus is slow in response. A reason is considered as follows.
In a typical active matrix liquid crystal display apparatus having TFTs as the switching elements as described above, an image is displayed in the following manner. A TFT connected to a pixel electrode is put into the selected state when a gate signal is sent to the gate electrode of the TFT via a gate line. If a source signal is sent to the source electrode of the TFT via a source line while the TFT is in the selected state, a charge is written to the pixel electrode via the drain electrode connected to the source electrode, whereby a pixel capacitor (=LC capacitor+storage capacitor CS) is charged with a predetermined amount of charge. With this charging, liquid crystal molecules of the liquid crystal layer are made to take a desired aligned state. The storage capacitor functions to hold the charge amount charged in the LC capacitor. The liquid crystal layer will be short in life if it is driven with a DC voltage. For this reason and others, the source signal sent from the source electrode is reversed in polarity every charging (frame reversal) to enable drive of the liquid crystal layer with an AC voltage.
Idealistically, the charge amount charged in the pixel capacitor is desirably constant until the TFT is put into the selected state next time. The following equation is established among the charge amount Q, the pixel capacitance Cpixel (=LC capacitance CLC+storage capacitance CS), and the voltage VLC applied to the LC capacitor, which is equal to the potential difference VS between the potential of the source signal and the potential of the counter electrode when the counter electrode is grounded.Q=Cpixel·VS
There is a phenomenon that the dielectric constant of liquid crystal molecules increases when response from white display to black display is attempted in a normally-white TN mode, for example. That is, Cpixel(white)<Cpixel(black). Therefore, when a predetermined voltage supposed to turn the state to black display is applied to the pixel capacitor in the white display state, the voltage actually applied to the pixel capacitor is lower than the predetermined voltage due to the increase of the dielectric constant of the liquid crystal molecules (hereinafter, this phenomenon is called “voltage variation”), and thus no black display state is attained. The black display state will eventually be attained by repeating the application of this voltage (charging) several times. This is the reason why the response of liquid crystal molecules is apparently slow. Theoretically, this voltage variation occurs in every response between gray-scale levels, that is, in any moving-image display. Therefore, every response between gray-scale levels is slow due to the voltage variation.
To solve the above problem, JP 3-35218A, for example, discloses a technology of capacitance coupling in a CS-on-gate type TFT liquid crystal display apparatus as follows. Two values are newly added to the conventional two-value gate signal (high potential for turning ON the TFT and low potential for turning OFF the TFT) of the gate line, to obtain a four-value signal. The newly added two values constitute a modulation signal, which is used for exchange of charge with the storage capacitor to thereby ensure application of a predetermined voltage to the pixel capacitor. In this way, the voltage variation can be reduced and, as a result, the response of liquid crystal molecules can be made faster.
However, the capacitance coupling described above has a drawback that it is not possible to reverse the pixels adjacent in the gate line direction in polarity from each other and thus flickering is likely to appear. To overcome this drawback, JP 11-218736A discloses a technology as follows. The storage capacitors of pixels arranged in the gate line direction are alternately connected to one gate line and the vertically adjacent gate line. This structure is combined with H line reversal drive in which pixels adjacent in the source line direction are reversed in polarity. By this combination, all pixels are reversed in polarity from the adjacent pixels in both the vertical and horizontal directions, and thus flickering can be reduced.
JP 4-145490A discloses the capacitance coupling for a CS-on-common type liquid crystal display apparatus, in which a storage capacitor line is driven independently for each gate line so that a modulation signal is superposed on the LC capacitor, to thereby obtain substantially the same effect as that obtained by the CS-on-gate type.